testsuite: LB viscous test guard for EXTERNAL_FORCES.

[KaiSzuttor]

Fixes #2334

[codecov]

Codecov Report

Merging #2335 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2335    +/-   ##
=======================================
+ Coverage      71%     71%   +<1%     
=======================================
  Files         380     381     +1     
  Lines       18842   18918    +76     
=======================================
+ Hits        13497   13589    +92     
+ Misses       5345    5329    -16

Impacted Files	Coverage Δ
src/core/virtual_sites.cpp	80% <0%> (-2%)	⬇️
src/script_interface/lbboundaries/LBBoundary.hpp	75% <0%> (-1%)	⬇️
src/core/grid_based_algorithms/lb.hpp	71% <0%> (-1%)	⬇️
src/core/collision.cpp	81% <0%> (-1%)	⬇️
...c/core/unit_tests/ParallelScriptInterface_test.cpp	100% <0%> (ø)	⬆️
...rc/core/utils/serialization/CUDA_particle_data.hpp	100% <0%> (ø)	⬆️
src/core/utils/math/tensor_product.hpp	100% <0%> (ø)	⬆️
src/core/cuda_init.cpp	0% <0%> (ø)	⬆️
src/core/MpiCallbacks.cpp	100% <0%> (ø)	⬆️
src/core/PdbParser.cpp	0% <0%> (ø)	⬆️
... and 30 more

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